1. Field of the Invention
The present invention is directed to impact modified styrenic polymers, and in particular to high impact modified polystyrene resins, in solid form, that exhibit improved environmental stress crack resistance properties.
2. Description of the Prior Art
A number of applications for impact modified polymers is in areas where there is a high potential for contact with oily or fatty substances. Polyisobutylene (PIB) has been added to high impact modified polystyrene (HIPS) materials during their manufacture to make specialty products with improved stress crack resistance.
For example, U.S. Pat. No. 5,543,461 discloses that HIPS and other impact modified styrenic polymers such as ABS and MBS tend to suffer from environmental stress cracking. On extended exposure to oily or fatty substances, the HIPS tends to craze, then crack and finally break. This disclosure proposes that environmental stress crack resistance (ESCR) of impact modified styrenic polymers may be increased by increasing the particle size of the impact modifier and including a low molecular weight polybutene.
U.S. Pat. Nos. 5,861,455 and 6,613,837 discloses a HIPS material that allegedly exhibits improved environmental stress crack resistance that is formed by utilizing polybutadiene, polyisoprene, and copolymers thereof with styrene, having a Mooney viscosity exceeding about 35 and a gel content of up to about 28%. The disclosure suggests using a combination of lubricant additives to obtain a HIPS material with higher ESCR values than either additive alone could provide.
U.S. Pat. No. 7,294,676 discloses styrenic resin compositions that include a rubber modified styrene maleic anhydride copolymer and polybutene. The polybutene ranges from 0.1 to 8% by weight and has a number average molecular weight from 900 to 2500. The rubber ranges from 4% to 20% by weight and has a particle size from 0.1 micron to 11 microns.
U.S. Patent Application Publication 2006/0178543 discloses a foamed sheet containing a polymer composition including a polymer formed by polymerizing a mixture including styrenic monomers; maleate-type monomers; elastomeric polymers; and low molecular weight polymers that include one or more monomers according to the formula CH2CR3R2, where R3 is H or a C1-C3 alkyl group and R2 is a C1-C22 linear, branched or cyclic alkyl or alkenyl groups. The foamed sheets can be thermoformed into containers suitable for use in microwave heating of food.
U.S. Patent Application Publication 2008/0081137 discloses a polymer composition that includes a blend of a rubber modified styrene maleic anhydride copolymer, HIPS, a rubber modified styrene methyl methacrylate copolymer and polybutene.
U.S. Patent Application Publication 2006/0160949 discloses a thermoplastic sheet containing a polymer formed by polymerizing a mixture including styrenic monomers; maleate-type monomers; elastomeric polymers with Mn greater than 12,000; and low molecular weight polymers, having Mn of from 400 to 12,000, that include one or more monomers according to the formula CH2CR3R2, where R3 is H or a C1-C3 alkyl group and R2 is a C1-C22 linear, branched or cyclic alkyl or alkenyl groups.
An issue with the foregoing compositions is that the polybutene is a very thick liquid that requires specialized equipment for handling due to its very high viscosity and sticky consistency. It would be beneficial for polystyrene compounders to be able to add polybutene themselves as needed to tailor standard HIPS products to achieve the level of ESCR needed for specific applications. However, polystyrene compounders do not typically have the specialized equipment needed to handle polybutene in this viscose liquid form.
Thus, it would be desirable to provide polybutene to polystyrene compounders in a form that would enable them to conveniently blend polybutene into HIPS and other impact modified styrenic resin compositions.